Mapping G1 phase by the structural morphology of the prematurely condensed chromosomes

The object of this study was to develop a map of G1 phase on the basis of the progressive changes taking place in the morphology of the prematurely condensed chromosomes as the cells traverse through G1 and then use this technique to determine the cell cycle location of normal and transformed cell populations in plateau phase. The morphology of the prematurely condensed chromosomes (PCC) of G1 cells in random populations was found to be highly variable. For a better understanding of the relationship between the morphology of the G1‐PCC and their position within G1 phase, synchronized populations of Chinese hamster ovary (CHO) cells in early, mid‐, and late G1 phase were fused with mitotic cells. Early G1 cells resulted in highly condensed G1‐PCC, while late G1 cells gave very extended G1‐PCC. Mid‐G1 cells resulted in PCC of intermediate condensation. To test the validity of these criteria for mapping the position of a cell in the cell cycle, synchronous G1 cell populations were treated with a variety of metabolic inhibitors. Cycloheximide and actinomycin D were shown to block cell in early G1 phase, while excess thymidine and hydroxyurea blocked cells in early S phase. The results presented here indicate that, upon reaching plateau phase, normal cell populations (BALB‐C mouse 3T3, human PA‐2, and WI 38) stop in early G1, while most cells in transformed cell lines (CHO, HeLa, and mouse SV‐3T3) accumulate in late G1.